1. Field of the Invention
The present invention relates to an electric double layer capacitor, particularly an electric double layer capacitor excellent in the operation reliability and an electrode assembly and a current collector therefor.
2. Discussion of Background
The electric double layer capacitor is based on a principle to store electric charge in an electric double layer formed at the interface between an electrode and an electrolyte. In order to improve the capacitance density of an electric double layer capacitor, it is common to employ for the electrode, a carbonaceous material such as activated carbon or carbon black having a high specific surface area or fine particles of a metal or conductive metal oxide. For efficient charging and discharging, the electrode is bonded to a current collector made of a foil or a layer having low resistance, such as a metal or graphite. As the current collector, it is common to employ a valve metal such as aluminum or a stainless steel such as SUS304 or SUS316L, which is electrochemically highly corrosion resistant.
As an electrolyte for the electric double layer capacitor, an organic electrolyte and an aqueous electrolyte are available. However, an attention has been drawn to an electric double layer capacitor employing an organic electrolyte, as the operating voltage is high, and the energy density in the charged state can be made high. In a case where an organic electrolyte is employed, if water is present in the cell of an electric double layer capacitor, the performance tends to deteriorate due to electrolysis of the water. Accordingly, the electrode is required to have water sufficiently removed, and it is common to apply drying treatment by heating under reduced pressure. As the electrode for an electric double layer capacitor, activated carbon is mainly used as the main component. However, active carbon is usually in a powder form. Accordingly, it is common to mix it with a binder containing a fluorine-containing resin such as polytetrafluoroethylene (hereinafter referred to as PTFE), followed by molding into an electrode of a sheet form, which is then electrically connected to a current collector via a conductive adhesive layer to form an electrode assembly. In order to improve the adhesion between the current collector and the electrode made of a carbonaceous material, JP-A-57-60828 or JP-A-57-84120 proposes an electrode assembly using a highly etched aluminum foil as a current collector. When such a highly etched aluminum foil is used as a current collector, and it is dipped in a slurry comprising a carbonaceous material such as active carbon, a binder and a solvent, or coated with such a slurry, followed by drying to obtain an electrode assembly, the adhesion will be improved over a case where a smooth aluminum foil or an aluminum foil having the surface roughened by e.g. sand blasting, is used. However, the obtained electrode assembly has poor strength and is likely to break in the process for producing the electrode assembly or in the process for preparing a capacitor by laminating the electrode assembly and a separator.
Further, it has been proposed to use an etched foil for an aluminum electrolytic capacitor, as the current collector for an electric double layer capacitor. However, the foil for an aluminum electrolytic capacitor is intended, as the basic purpose of the product, to have a strength sufficient to be rolled together with a separator and to provide a high capacitance. Whereas, the current collector for an electric double layer capacitor is required to have a high bonding strength with an electrode composed mainly of a carbonaceous material and to have a strength durable in various process steps for forming an electrode assembly by integrating it with the electrode, which is essentially different from the basic purpose of the foil for an aluminum electrolytic capacitor. Accordingly, even if the foil for an aluminum electrolytic capacitor is used for a current collector of an electric double layer capacitor, good properties as an electric double layer capacitor can hardly be obtained, and the productivity will also be poor.